The invention relates to a valve, especially a high-pressure control valve.
Such high-pressure valves are used in chromatography (supercritical fluid chromatography-"SFC" for short) at pressures of up to several hundred bar. In chromatography they act as counter pressure control valves which maintain a specific predeterminable pressure in a separating column. They are, however, also used in extraction processes using supercritical media and generally in systems in which it is necessary to monitor and to control the pressure patterns of gases, liquids and supercritical media over time. For operation of the valve it is already known to use an electromagnetic drive that is capable of moving the valve body in the opening direction against a spring force. Valves of that type require the starting position of the valve to be adjusted manually for a specific working range. This adjustment has to be made very carefully and is therefore very time-consuming. In addition, the closing spring force must be such that, on the one hand, sealing closure is possible and, on the other, damage to the valve seat or to a seal in the valve seat is avoided. The closing force of the spring may at most be as great as the magnetic force available for opening.
Owing to the transition in the valve from a region of high pressure to a region of low pressure, when the supercritical medium expands there is a risk that ice will form, which can lead to considerable impairment of the control function of the valve or to destruction of the valve seat. On the other hand, however, it is necessary in the case of preparative work to be able to collect the medium in solid form (ice). The impairment of the functioning of the valve in the case of ice formation is attributable to the fact that ice forms between the valve seat and the valve body, preventing the valve from closing. This applies in the same way to other solid particles that may be contained in the investigation medium.
In electromagnetic control valves in which closing is effected by a spring, the force of the spring is so adjusted to "normal operation" that on the one hand reliable functioning is achieved and on the other hand damage to the valve is avoided. This spring setting is not generally sufficient, however, to overcome ice or other solid particles on closing, so that the valve-control function is disrupted. If the end faces of the valve seat and the valve body oppose one another and rest one on the other in the closed position, the ice or the like can occur as a layer, which means that even greater adjusting forces would be needed to push away those solid particles.
Needle valves, which pose fewer problems when solid bodies and ice are present, are also known, but they are sensitive mechanically.
In order to prevent the formation of ice on the counter pressure control valve and hence to avoid impairment of its functioning, arrangements having multi-stage, generally two-stage, expansion with a plurality of valves are already known, the control valve being arranged in the region of the first expansion stage. The pressure difference in that stage is such that icing cannot occur. This multi-stage arrangement does, however, require additional expenditure.
In order to prevent icing, it is also possible to provide heating means in the region of the control valve. In practice, however, for reasons of space, this is generally possible only with difficulty. In addition, with the given geometry, the heat transfer is generally too low. Furthermore, preparative work cannot be carried out using heated valves.
FR-A-2,425,559 discloses a valve which is opened by means of a piezo-crystal. The valve is closed by means of a mechanical spring. This valve is used for pressures below 1 bar, especially in the introduction of controlled gas streams into a vacuum. This valve is not suitable, however, for use as a high-pressure control valve for pressures of several hundred bar, for example up to 800 bar, since a mechanical spring would not withstand such pressures.
DE-A-3,608,550 describes a three-way valve. That valve has a housing into which at least three valve channels open. Using a piezo-electrically adjustable sealing member, the working channel can be connected either to one valve channel or to the other. This three-way switching valve cannot be used as a counter pressure control valve at pressures of several hundred bar.
A similar three-way switching valve is described in U.S. Pat. No. 3,386,472. In the case of that valve, different channels in the valve are connected with the aid of a magnetically actuatable sealing member. This three-way valve cannot be used as a high-pressure control valve.